Use ephemeral S3 buckets for E2E tests

[ameukam]

Use S3 buckets created during the lifecycle of a test instead of a
static one.

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[rifelpet]

We'll need a second bucket for the cluster's oidc discovery store, we can reuse this logic but it will need a policy that allows anonymous read access.

[rifelpet]

We might want to add a random or timestamp suffix. S3 has a delay before a deleted bucket's name can be reused, and if a boskos account is quickly reused between kops jobs then the bucket creation could fail.